Inside of cell becomes more and more positive until electrial force repels positive Na+ ions at equilibrium potential the diffusion forces pushing Na+ ion in is exactly counterbalanced by the electrial force pushing them out.

Equilibrium potential is the membrane potential we measure when membrane is permeable to that ion alone. ex if only K+ ion channels in membrane, and 20times more K+ inside we get a membrance potential of -80mv. if only Na+ channels in membrane and 10 times more Na+ inside membrance potential is +61.5mv

Resting membrance potential

resting membrane potential of -65mv is close to ,but not at , equilibrium potential for K+, at rest membrane has mostly K+ channels open.